Process for the preparation of alumina abrasives

ABSTRACT

The present disclosure is directed to a method of converting the preferred hydrated alumina oxide, boehmite, into a green particle which is then sintered to form an abrasive alumina particle. The process involves dispersing boehmite in water at a slightly elevated temperature with an acidic pH, then seeding it with alumina seed particles and drying it, then crushing to desired shapes and sizes, calcining the dried precipitate and sintering the calcined. This forms a high quality alumina abrasive grains. The apparatus used for sintering incorporates an elongate hollow tube, an insulative sleeve thereabout to define an elevated temperature zone, and a microwave generator coupled through a wave guide into a microwave cavity incorporated in the tube. The particles are moved through the tube at a controlled rate to assure adequate exposure to the microwave radiation.

BACKGROUND OF THE DISCLOSURE

Microwave heating has demonstrated itself to be a powerful technique forsintering various ceramics, especially through the past decade.Microwave heating may decrease the sintering temperatures and timesdramatically, and is economically advantageous due to considerableenergy savings. However, one of the major limitations is the volumeand/or size of the ceramic products that can be microwave sintered,because of an inhomogenous microwave energy distribution inside theapplicator which often results in a non-uniform heating. Considerableresearch has gone into making microwave sintering technology viable, butbatch processing was always a handicap.

Continuous microwave sintering of alumina is a newly developed process.The principle of the continuous microwave sintering furnace is shown inFIG. 1. The microwave applicator is designed to focus the microwavefield in the central area as uniformly as possible. A long cylindricalceramic hollow tube contains the green alumina which is fed into themicrowave applicator at a constant feed speed. As the alumina enters themicrowave cavity, it is heated and gradually sintered while passingthrough the microwave zone. The heating rate, sintering time and coolingrate are controlled by the input microwave power, the feeding speed, andthe thermal insulation surrounding the heated alumina. The ceramichollow tube is also rotated during processing for uniform and homogenousheating. As the green alumina passed through the high temperature zone,the particles are sintered entirely. Since the ceramic hollow tube ismoved relatively continuously in the axial direction during theprocessing, there is virtually no limitation to the length of theproduct that can be processed by this technique. Consequently, it ispossible to scale-up the volume of the ceramic products to be microwavesintered by this technique to provide a continuous process.

This disclosure proves the continuous microwave sintering technique forlarge quantities of commercial alumina grains with properties requiredfor use as an advanced abrasive grain.

The disclosure also is directed to a novel synthesis method for themanufacture of alumina based sol gel abrasive utilizing the newlydeveloped microwave processing. The process offers a faster, energyefficient route to manufacture abrasive alumina grains. Grains preparedby this method exhibited micro hardness above 2200 kg/mm² 98%theoretical density, crystalline uniformity and average size less than0.5 microns with high abrasion properties.

One aspect of this invention relates to improved preparation of aluminaabrasives. The alumina particles are used for both coated and bondedabrasives. They are conventionally produced using arc furnace meltingtechnology with either calcined bauxite or calcined alumina as thestarting material and at temperatures above 2000° C.

These conventionally prepared alumina abrasives normally have grainsizes above 150 microns. The conventional process for the production ofalumina abrasive grains is through fusion using the known arc furnace.The calcined alumina is melted at temperatures above 2000° C. orcalcined bauxite is fused at temperatures above 2000° C. in presence ofcarbon to reduce its impurities such as titania, iron oxide, silica etc.

In both the above cases, the molten material is cooled, crushed,magnetically screened and graded to obtain the various grit sizesrequired for abrasive grain preparation. Since the process requiresmelting of the raw material, the process becomes highly energyintensive; worse, the crystal size of the resulting abrasive grain ismore than 150 microns in size.

Subsequently, another process was developed through sol gel routeyielding abrasive grains with small crystal sizes of about 0.5 micronsconsuming low energy. In this route, the major steps so far employedinclude dispersing the alumina oxide (Boehmite) in water to form aluminain sol state, addition of seeds like alpha alumina either by millingusing alumina grinding media or direct addition of micronised alphaalumina particles. Then, selected additives such as magnesia, titania,yttria, etc., added; the material is dried, crushed and sintered usingconventional sintering furnace. The conventional sintering furnace istypically an electric or oil or gas fired batch kiln with a rotary kiln.The heating rate is about 20° C./minute as the system applies externalheating to the material.

As a rule of thumb, the performance of the abrasives with the samehardness, toughness and density improves with decrease in grain size. Itis possible to achieve very small grain sizes with high hardness,toughness and density, using the combination of sol get and microwaveprocesses thereby improving the abrasive characteristics when comparedto the conventional process. This process requires lower temperature,around 1400° C.

By the use of the process of the present invention, it is possible toprepare a new variety of alumina abrasive grains at considerably lowerenergy with small crystal size, high hardness and density. The processof the present invention also involves use of a microwave sinteringtechnique in which higher heating rates are employed to form finerparticles than conventional products. In the process of the invention,microwave heat is generated internally within the material instead oforiginating from external heating sources and is a function of thematerial being process. It is seen that as the temperature increasesabove a certain point, the dielectric loss begins to increase rapidlyand the material begins to absorb microwaves more efficiently. This alsoraises the temperature very rapidly. In many cases the heating rates areas high as 300° C./minute. Both batch and continuous processing systemscan be employed.

The raw material used in the process of the present invention isboehmite dispersed in water with a concentration in the range of 15 to30 weight percentage. The pH is adjusted between 2 to 4 by controlledaddition of an acid such as nitric acid and peptized. The additives suchas iron, silicon, titanium, magnesium, yttrium, neodymium, lanthanumetc., are added as their hydroxides, oxides or nitrates. The amount ofadditives may vary from 0.1 to 7 weight percentage when considered astheir oxides. Seeding involves sub micron alpha alumina seeds in theamount of up to 1.5% by weight. Control drying chemical additives may beadded in the range of 0.1 to 1.0% to ensure uniform heating of themixture and to aid pore free drying. The gel prepared is dried at thetemperatures of 600° to 800° C. The dried gel is crushed to the requiredsize giving allowance for the shrinkage during sintering. The shape ofthe grains is modified to suit the end use by selection of a suitablecrushing system.

The graded gel is then calcined in the temperature range of 300° to 900°C. for about 30 minutes to drive away the volatiles. The calcined grainsare then sintered using microwave technology at temperatures less than1500° C.

Another object of the present invention is to provide an improvedprocess for the preparation of alumina abrasive grains having lowcrystal sizes, high hardness and toughness and density overcoming thedrawbacks of the hitherto known processes.

Yet another object of the present invention is to produce an improvedprocess for the preparation of alumina abrasive grains having MicroVickers hardness above 2100 kg/sq. mm, 90% theoretical density withcrystalline uniformity and average crystal size less than 0.6 microns.

DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained and can be understood indetail, a more particular description of the invention, brieflysummarized above, may be had by reference to the embodiments thereofwhich are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic representation of a microwave system suitable forcontinuous sintering;

FIG. 2 shows alumina grit in microphotograph; and

FIGS. 3 and 4 are microphotographs of alumina grit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The microwave oven employed (equipped with a power control and a timer)produces microwave energy of 2.45 GHz frequency and power output of 900W. The calcined grain is placed in the closed insulating chamber, calledthe microwave cavity. The insulating material is an aluminum silicatebased material. An inner sleeve of porous zirconia is also included. Thesystem reduces heat loss while maintaining high temperatures. A sheathedthermocouple is introduced for temperature measurement, and placed nearthe sample. As a precaution, the body of the oven is provided withwatercooled tubes. This microwave oven procedure provides batch orcontinuous processing of alumina abrasive grains. For a continuousoperation, the material is added to the top of a sintered alumina/orfused quartz tube in the microwave field. The material for sintering iscontinuously fed from top and sintered alumina grains are drained at thebottom of tube at a controlled rate.

The grit manufacturing process is set out in the examples given belowwhich are provided by way of illustration only and should not beconstrued to limit the scope of this present invention.

EXAMPLE 1

400 g of boehmite was dispersed in 2.4 liters of hot water at 80° C.,and the pH was brought down to 2.5. To this, 3.46 g of ferric oxide wasadded in the form of its hydroxide followed by addition of 0.62 g oftitania as its hydroxide, and also 0.78 g of precipitated silica and0.51 g of calcia. 2.0 g of polyethylene glycol was added. Seeding wasdone using 4.5 g of submicron alpha alumina seeds. This was followed bythe addition of 30.8 g of magnesium nitrate as a solution and thedispersion was continued for an additional 20 minutes more. This wasdried at 80° C. for 30 minutes. Calcined gel was then sintered in themicrowave oven at a temperature of 1400° C. for 45 minutes.

The product showed an average crystal size of 0.5 microns, specificgravity of 3.91 g/cc, micro Vickers hardness of 2205 kg/sq. mm, with anabrasion index of 85.

EXAMPLE 2

400 g of boehmite was dispersed in 3 liters of water, and the pH wasbrought down to 2 followed by the addition of 1.5 g of titania. It wasseeded using 3.0 g of submicronised alpha alumina seeds followed by theaddition of 46.2 g of magnesium nitrate as a solution in water. Afterdispersing again for half an hour, the gel was dried at 70° C. for 30hours. The dried gel was crushed, graded and calcined at 500° C. for anhour. These grains were then sintered in the microwave oven at atemperature of 1450° C. for 15 minutes. The product showed an averagecrystal size of 0.6 microns, specific gravity 3.84, micro Vickershardness of 1863 kg/sq mm and abrasion index of 80.

EXAMPLE 3

400 g of boehmite was dispersed in 2 liters and pH brought down to 3.Both yttria and lanthanum oxide were taken in equal quantities of 2.25 gand dissolved in dilute nitric acid and added followed by 3 g ofsubmicron sized alpha alumina seeds. The gel was then dried at 80° C.,crushed and graded. The graded material was then calcined at 600° C. for30 minutes followed by microwave sintering at 1450° C. for 30 minutes.

The abrasive grain had an average crystal size of 0.5, specific gravity3.84, micro Vickers hardness of 1863.25 kg/sq. mm and an abrasion indexof 95.

EXAMPLE 4

400 g of boehmite was dispersed in 2 liters of water at 80° C., the pHwas brought down to 3 by the addition of nitric acid, while beingdispersed, the additives 2.25 g of yttria, 2.25 g of lanthanum oxide and2.25 g of neodymium oxide were added. 2 g of polyethylene glycol wasadded followed by alpha alumina seeds of 3 g. The gel was then dried at70° C. This dried gel was crushed, graded and calcined at 600° C. for anhour. The calcined gel was then sintered at 1400° C. for 45 minutes.

The sintered grains where of average crystal size 0.4 microns, specificgravity 3.85 gm/cc, Micro Vickers Hardness 1897.3 kg/sq. mm and abrasionindex of 95.

EXAMPLE 5

400 g of boehmite was dispersed in 1.8 liters of water and the pH wasbrought down to 2.5. To the above solution, we added 2.25 g of yttria,2.25 g of lanthanum oxide, 2.25 g of neodymium oxide, 1.15 g of cobaltnitrate and 3 g of aluminum nitrate followed by 1.5 g of polyethyleneglycol. The dispersion was continued and 4.5 g of alpha alumina seedswere added and the dispersion was stopped after another 20 minutes. Thiswas then dried at a temperature of 80° C. for 20 hours and the dried gelwas crushed, graded and calcined at 500° C. for 1 hour. This was thenmicrowave sintered at 1450° C. for 30 minutes. These showed an averagecrystal size of 0.5 micron, specific gravity 3.94, Micro VickersHardness 2387.76 kg/sq. mm and abrasion index of 100.

EXAMPLE 6

400 g of boehmite was dispersed in 1.8 liters of water, the pH wasbrought down to 2.5 by adding nitric acid, 4.5 g of yttria, 2.25 g oflanthanum oxide and 1.15 g of cobalt nitrate; then we added 3 g of alphaalumina seeds and 1.5 g of polyethylene glycol. After dispersing for 20minutes, it dried at 80° C. The gel so dried was crushed and graded.This was then calcined at 500° C. for an hour and microwave sintered at1400° C. for 30 minutes.

The abrasive grains so produced showed an average crystal size of 0.4microns, specific gravity 3.86 gms/cc, Micro Vickers Hardness 1863 andan abrasion index of 98.

For some of the abrasive grains prepared by the process of theinvention, the specific gravity and micro hardness values are providedbelow in the examples listed.

    ______________________________________                                               MW Sintering                                                                              Residence Time                                                                            Specific                                                                             Hardness                                Example                                                                              Temp °C.                                                                           Minutes     gravity                                                                              VH                                      ______________________________________                                        V (1)  1350        30          3.72   1880.25                                 V (2)  1350        60          3.88   1880.25                                 V (3)  1400        15          3.82   1897.31                                 V (4)  1400        30          3.85   1863.43                                 V (5)  1400        45          3.85   1880.25                                 V (6)  1450        15          3.88   2316.66                                 V (7)  1450        30          3.94   2387.76                                 V (8)  1500        5           3.70   1036.52                                 V (9)  1500        15          3.96   2316.66                                 ______________________________________                                    

Abrasion Index

This is a measure of abrasiveness. It is determined by using pin on discsystem, a system which includes a rotating disc with a channel to holdthe grains to be tested. An aluminum cylinder of 20 mm diameter and 25mm length is fastened to a free rotating holder. A constant load of 10.5N is applied on the sample. The disc is rotated at a fixed speed of 60RPM. A slurry containing 30 weight percent abrasive grains of grit 36 isplaced in water/glycerine medium. The loss in weight of the sample isnoted for an hour at intervals of 15 minutes.

    ______________________________________                                        Example      Abrasion Index                                                   ______________________________________                                        V (1)        63                                                               V (2)        66                                                               V (5)        95                                                               V (7)        100                                                               V (10)      94                                                               ______________________________________                                    

Microwave Sintering Operation

The feed material is a gel solution derived from alumina grit withaverage particle size of about 0.6 to about 1 mm. The dried grit ispacked into a high purity alumina tube (30 mm in diameter and 900 mm inlength) 12 which is held by a metal clamp 14 connected to the shaft ofthe rotating motor 16. The tube 12 is inserted into the microwaveapplicator 18 with a middle portion located in the central area 20 ofthe cavity. At the beginning, the tube is stationary in the originalposition and is held while rotating only, without vertical feedingmovement. Microwave power is introduced to the applicator 18 andcontrolled to achieve the desired high heating rate (50° to 300°C./min). When the sample temperature reaches the set temperature, thefeeding motor 22 is started to feed the tube at the desired speed (about2 mm per minute). The temperature of the sample is monitored by an IRpyrometer (Accufiber Inc.), and is controlled by adjusting the incidentmicrowave power. Sintering temperature and time can be varied from 1350°to 1500° C. and 45 to 5 minutes respectively. Parallel experiments fromconventional furnace are reported to compare the results of the twoprocesses.

Going over the apparatus in FIG. 1 in some detail, the microwave system10 incorporates a microwave generator 22 which forms the microwaveradiation at some extremely high frequency which is conveyed by a waveguide 24 to the microwave cavity. The cavity is defined on the interiorof an insulative sleeve 26. The sleeve 26 prevents heat loss through thetube 12 as will be explained. The microwave cavity incorporates thecentral area 20 previously mentioned. In the central area, the materialis heated in a first zone 28 and reaches the maximum or sinteringtemperature in an intermediate zone 30. Zone 30 is continuous with thezone 28. As the product moves downward, it enters into the zone 32 wherecooling begins. There is a discharge zone 34 at the lower end. Thesintered material is delivered through the lower end 36. For the sake ofcontrolling the flow rate, a valve 38 is affixed at the lower end tometer the delivered product. At the upper end, the tube is hollow at thetop end and the raw ingredients are introduced through the upper end 40.The collar or clamp 14 fastens on the exterior and preferably leaves thetop end open for material to be added. The clamp 14 holds the tube 12for rotation when driven by the motor 16 as mentioned.

An adjacent upstanding frame 42 supports a protruding bracket 44 alignedwith a bottom bracket 46. The brackets 44 and 46 hold a rotating screw48 which serves as a feed screw. A movable carriage 50 travels up anddown as driven by the screw. The screw 48 is rotated by the feed motor22 shown at the lower end of the equipment. Rotation in one direction orthe other causes the carriage 50 to move up or down as the case may be.

The microwave system is provided with an adjustable power control 56 anda timer 58. The timer is used in batch fabrication which is normallysimply switched on for continuous sintering. Attention is momentarilydiverted to one aspect of the tube 12.

It preferably is a dual tube construction with a tube 60 fitting snuglyinside the outer tube 12. This defines an internal cavity through whichthe porous particulate alumina is added at the top 40. It travels alongthe tube at a rate determined by the rate at which the valve 38 isoperated so that the material is maintained in the hottest zone 30 for acontrolled interval. For instance, the rate of flow down through thetube can be increased or decreased by throttling the flow through thevalve 38. This assures that the material remains in the hottest portionof the microwave cavity 30. By rotating the tube continuously andcontinuing a feed through the tube 12 which causes gradual, relativelydownward linear motion, the particles are processed as appropriate formicrowave sintering in a batch. By rotating without feeding the tube 12through the cavity 20, but with controlled particulate flow through thetube 12 and valve 38, continuous operating is provided. Otherwise, asingle batch can be put in the tube and the tube advanced by the feedmechanism to heat one batch.

The morphology and microstructure of the samples were characterized bySEM, the densities of the sintered samples were measured by theArchimedes method, and the Vickers hardness was measured by MicroIndentation Method.

The grit morphology of the starting and sintered grit samples is shownin the attached microphotographs of FIGS. 2, 3 and 4. The shape of thegrit did not change, but the average particle size of the sinteredsample decreased about one third because of the shrinkage of grit duringthe sintering. It was expected that the grit particles would bindtogether tightly after the sintering. However, the results showed thatthere was no or very weak bonding between the particles. The sinteredgrit remains separated. This is important as it makes it possible tofeed the grit into the alumina tube continuously with the automaticfeeder during the microwave sintering. Thus, processing of large amountsfor commercial production can be achieved.

The quality of the microwave sintered alumina abrasive grit mainlydepends on the sintering temperature and time. During the continuousmicrowave sintering process, the temperature is controlled by microwavepower, and the sintering time (actually, this is the residence time ofthe grit samples in the high temperature zone) depends on the height ofthe high temperature zone and the feeding speed. Theoretically, higherfeeding speed will lead to a higher product output, but has to beoptimized for each material to accomplish high quality products. Theuniform high temperature zone is about 30 mm long in the microwaveapplicator. In this case, the residence time of the sample in the hightemperature zone was about 15 minutes at a feeding speed of 2 mm/minute.

A large batch of commercial alumina abrasive grit was processed by anewly developed continuous microwave sintering technique. The quality ofthe microwave sintered products depends on the sintering temperature andsintering time. During the continuous microwave sintering processing,the sintering time can be controlled by the feeding speed of the greenparticles. As seen from the results the microwave sintered product givesmuch high abrasive index and hardness values with uniformly controlledmicro structure.

As used in the claims which follow, the term "aluminum oxide" includes avariety of aluminum oxide forms. While boehmite is named, it alsoincludes anhydrous forms also.

While the foregoing is directed to the preferred embodiment, the scopethereof is determined by the claims which follow.

We claim:
 1. An improved process for the preparation of alumina abrasivegrains which comprises the steps of:(a) dispersing aluminum oxide inwater at a temperature in the range of 40° to 80° C.; (b) adjusting thepH of the dispersed solution to an acidic range of about 4 or less; (c)seeding the solution by adding submicronized alpha-alumina particles toassist in precipitation; (d) crushing the recovered precipitate; (e)calcining the crushed precipitate at a temperature in the range of about300° to 900° C. for a period in the range of about 30 to 120 minutes;and (f) sintering the calcined precipitate by microwave at a temperaturebetween about 1200° and 1500° C. for a period of about 5 to 60 minutes.2. An improved process as claimed in claim 1 wherein the pH ismaintained between about 2 to 4 by adding an acid to the water.
 3. Animproved process as claimed in claim 2 wherein additives selected fromthe group consisting of oxides, nitrates, or hydroxides are added to theacidified water.
 4. The product made by the method of claim
 1. 5. Themethod of claim 3 wherein the additives are added in an amount up toabout 7 wt %.
 6. The method of claim 1 wherein the amount of seed isadded in an amount up to about 2 wt %.
 7. The method of claim 1 whereinthe calcination is carried out at about 600° C. for at least about 30minutes.
 8. A method of making alumina abrasives which comprises:(a)dispersing boehmite in water at a temperature in the range of 30° to 80°C.; (b) adjusting the pH of the water to the range of 2 to 4; (c) mixingadditives in the water; (d) seeding the water with sub-micronized alphaalumina particles; (e) drying to remove water at a temperature in therange of 40° to 80° C.; (f) crushing the dried material to a selectedsize and shape; (g) calcining the crushed material in temperature rangeof 300° to 900° C. for a period ranging up to about 120 minutes; (h)sintering the calcined material by microwave at temperature below 1500°C. for a period up to about 60 minutes.
 9. The method of claim 8 whereinthe microwave radiation is about 2.45 GHz frequency.
 10. The method ofclaim 8 wherein the microwave energy is the sole heat source forsintering.
 11. The method of claim 8 wherein the step of sintering thecalcined material includes the preliminary step of pouring the materialinto a closed chamber in loose particulate form.
 12. The method of claim8 wherein the step of sintering the calcined material includes thepreliminary step of placing the crushed material in a microwavetransparent container prior to sintering, and then sintering bydirecting microwave energy through the container into the crushedcalcined material.